1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a printer, an apparatus for displaying recorded images, or a facsimile equipment, which forms visible images by developing electrostatic latent images formed on an image bearing member by an electrophotographic method, an electrostatic recording method, or the like. The invention also relates to a developing device used for the image forming apparatus.
2. Related Background Art
There has been known a developing device that bears a dry developer as a visualizing agent on a surface of a developer bearing member, and supplies this developer to the vicinity of a surface of an image bearing member that bears an electrostatic latent image so as to develop the electrostatic latent image into a visible image by applying an alternating electric field between the image bearing member and the developer bearing member.
In this respect, the developer bearing member described above is generally provided with a developing sleeve for use in many cases. The developer bearing member is, therefore, referred to as a xe2x80x9cdeveloping sleevexe2x80x9d in the following description. Also, since the image bearing member is generally provided with a photosensitive drum in many cases, the image bearing member is referred to as a xe2x80x9cphotosensitive drumxe2x80x9d in the following description.
As the above-mentioned development method, there is known the so-called magnetic brush development method whereby to form a magnetic brush on a surface of a developing sleeve in which a magnet which serves as magnetic field generating means is disposed. The magnetic brush is made of a developer (two-component developer) comprising a two-component system composition (carrier particles and toner particles), for example. This method is arranged to enable the magnetic brush to slide on or approach the photosensitive drum that faces the magnetic brush with a fine development gap between them, and then, to apply continuously the alternating electric field between the developing sleeve and the photosensitive drum (between S-D), hence performing the development by the repeated transfer and counter-transfer of the toner particles from the developing sleeve to the photosensitive drum (as disclosed in the specifications of Japanese Patent Application Laid-Open No. 55-32060 and Japanese Patent Application Laid-Open No. 59-165082, for example).
As shown in FIG. 5, the developing device 18 for the two-component magnetic brush development is provided with a developing container 18a which is divided into a developing chamber R1 and an agitating chamber R2 by a partition wall 19. In the developing chamber R1 and the agitating chamber R2, the agitating and conveying screws 23 and 24 serving as the agitating and conveying members are rotatively contained, respectively. At an opening of the developing chamber R1, the developing sleeve 25, which rotates in a direction indicated by an arrow xe2x80x9ccxe2x80x9d in FIG. 5 is arranged to face the photosensitive drum 3 which rotates in a direction indicated by an arrow a with a fine gap therebetween. The magnet 29 is fixedly arranged in the interior of the developing sleeve.
Also, a regulating blade 28 is arranged to face the developing sleeve 25 with a predetermined gap in order to provide the developer on the surface of the developing sleeve 25 in a form of a thin film.
In the developing container 18a, there is contained the developer 22 having the toner particles and the magnetic particles mixed for it. The mixing ratio of the toner particles and the magnetic particles (hereinafter referred to as the xe2x80x9cT/C ratioxe2x80x9d) is kept constant by droppig and supplying a toner in an amount matching that of the toner that has been consumed for development, from a toner storage chamber 21 that contains the toner to be supplied.
The dropped and supplied toner is agitated by the screw 24 in the agitating chamber R2 to be mixed with the developer 22 in the developing chamber 18a, and then carried. At this time, the developer 22 is conveyed along a longitudinal direction of the developing container in a direction opposite to a developer conveying direction in which the developer is conveyed by the conveying screw 23 in the developing chamber R1. The partition wall 19 is provided with openings on the front side and the back side respectively in FIG. 5. Through these openings, the developer is delivered.
Now, it is extremely important to maintain the T/C ratio of the two-component developer in the developing container 18a for the stabilized image output. Various methods have been proposed for the detection and maintenance of this ratio.
For example, detecting means is arranged on the circumference of the photosensitive drum 3 to irradiate light on the toner transferred from the developing sleeve 25 to the photosensitive drum 3. Thus, the toner supply amount is adjusted based on a transmitted light or a reflected light at that time. With the result thereof, the T/C ratio is detected. There is another method in which detecting means is arranged in the vicinity of the surface layer of the developing sleeve 25, and then, the T/C ratio is detected based on a reflected light when light is irradiated on the developer coated on the developing sleeve 25. Also, there is a method proposed and practiced, in which the T/C ratio is detected by the provision of a sensor in the developing container 18a to detect the change in the apparent magnetic permeability of the developer in a predetermined volume in the vicinity of the sensor by the utilization of the coil inductance.
However, the method of maintaining the T/C ratio at a constant value on the basis of an amount of the developing toner on the photosensitive drum 3 has a problem that the amount of toner on the photosensitive drum 3 is caused to change due to the factors other than the changes of the T/C ratio, such as the fluctuation of the gap between the photosensitive drum 3 and the developing sleeve 25 or the change in the electrical potential of a latent image, and as a result, the toner supply operation is made erroneous eventually. Also, the method of detecting the T/C ratio by the reflective light when the light is irradiated to the developer applied on the developing sleeve 25 has a problem that it becomes impossible to detect the exact T/C ratio if the detecting means is stained by the toner flown in all directions.
In contrast, the sensor that uses the method of detecting the T/C ratio by the detection of the change in the apparent magnetic permeability of the developer 22 within a predetermined volume in the vicinity of the sensor (hereinafter referred to the xe2x80x9ctoner concentration sensorxe2x80x9d) is not affected by the problem of the contamination due to the toner flown in all directions, while the costs of the sensor per se are lower. Therefore, this T/C ratio detecting means is best suited for the low-cost and smaller-space copying machine or an image forming apparatus, such as a printer.
The toner concentration sensor that utilizes the change in magnetic permeability of the developer as described above is arranged to determine that a T/C ratio of the developer becomes lower when the magnetic permeability of the developer within a predetermined volume becomes large, for example, and then, it causes the initiation of the toner supply. If, on the contrary, the magnetic permeability becomes smaller, it is determined that the T/C ratio of the developer is made higher, thus ceasing the toner supply. This sensor controls the T/C ratio of the developer in accordance with such sequence.
Nevertheless, the toner concentration sensor in the method of detecting the change in the apparent magnetic permeability of the developer within a predetermined volume as described above has a problem that if the bulk density of the developer itself, that is, the weight of the developer per unit volume, may be affected to present some changes, the apparent magnetic permeability of the developer may also change following the change in the bulk density, hence the sensor output being changed in accordance with the change in the magnetic permeability.
That is, even if a T/C ratio in a developing container remains the same, a bulk density in the developing container may be changed. This is because a change in an amount of a developer (carriers) in a predetermined volume in the vicinity of the toner concentration sensor causes a change in magnetic permeability at that time so that an output of the sensor is changed. As the result, even if a toner is not so consumed, the sensor may produce an output representative of a reduction of the toner so that a toner may be supplied. Conversely, even if an amount of the toner is reduced, the sensor may produce an output indicating that the toner is not reduced so that a toner supply cannot be performed.
In the former case, due to the excessive supply of toner, there is a problem that the image density becomes denser or the developer may overflow from the developing container due to the increased amount of developer along with the increase of the toner amount. There is also a problem that may be encountered that the toner flown in all directions or the like takes place due to the lowered amount of the electrostatic charge of the toner following the increased toner ratio in the developer.
In the latter case, there may be encountered a problem that the images are degraded or the image density becomes lighter due to the reduced amount of toner in the developer or the lighter image density or the like may take place due to the increased amount of the electrostatic charge of the toner.
After the detailed studies of the inventors hereof, it has been found that these problems are caused mostly by the change in the bulk density of the developer due to the change in the amount of the electrostatic charge of the toner in the developer.
If the fluctuation of the amount of the electrostatic charge of the toner is greater, it indicates the greater amount of change in force of repulsion between the developers. The greater the amount of the electrostatic charge of the toner, the greater becomes the force of repulsion between the developers. As a result, the space between the developers may spread more widely due to the greater force of repulsion to make the bulk density of the developer smaller.
Then, as another factor, it is found that the change in the temperature and humidity may exert an influence on the change in the amount of the electrostatic charge of the toner of the developer in a developer system and the developing device that adopts the aforesaid developing method. At a lower temperature with a lower humidity, the amount of moisture contained in the developer itself is reduced to increase the electrostatic charge on the toner generated by the contact between the toner and the carrier. Then, the repulsion between the developers becomes greater to make the bulk density of the developer smaller. On the contrary, if the temperature and humidity are high, the moisture content of the developer itself is increased to make it difficult to increase the electrostatic charge on the toner by the contact between the toner and the carrier. Thus, the repulsion between the developers becomes smaller to increase the bulk density of the developer.
In the specification of Japanese Patent Application Laid-Open No. 5-61353, for example, there is disclosed a method of changing a control voltage for controlling an intensity of a magnetic field generated by a toner concentration sensor in accordance with the temperature and humidity of the developer when the output of the toner concentration sensor fluctuates due to the changes in the aforesaid bulk density.
Also, in the specification of Japanese Patent Application Laid-Open No. 5-61353, there is a disclosure that the control voltage is changed to control the intensity of the magnetic field generated by the toner concentration sensor in accordance with the temperature characteristics of a varicap (variable capacitance diode) used for the oscillating circuit to generate the magnetic field of the toner concentration sensor. The temperature characteristics indicates the increase of the electrostatic capacitance if the temperature in the interior of an image forming apparatus becomes higher and indicates the reduction of the electrostatic capacitance if the temperature in the interior of the image forming apparatus becomes lower.
With the adoption of the aforesaid control method, it becomes possible to prevent the toner replenishment significantly from being erroneously operated due to the fluctuation of an amount of the electrostatic charge of the developer (toner) caused by the charge in temperature and humidity of the developer. Nevertheless, if toner should be prepared in ultrafine particles for obtaining recorded images in higher quality, there are still the problems yet to be solved as given below.
(1) After the output voltage of the toner concentration sensor is controlled under the and high humid environment, and after it is left intact for a long period of time, images are output for several thousands of sheets with the result that the T/C ratio of the developer is increased. Then, the density of copied image is increased for the one having a higher image ratio with the resultant toner adhesion to the background portion thereof.
(2) After the output voltage of the toner concentration sensor is controlled under the lower humid environment, several thousands of sheets of image are output from a source document having a lower image ratio on it. Then, the T/C ratio of the developer is lowered, and the reduction of the image density becomes lower considerably.
After the detailed studies made by the inventors hereof, it is found that these problems are caused by the following two phenomena.
One of the phenomena is brought about by the crushed toner in general use. The toner shape of each individual crushed toner is irregular, which easily results in the fluctuation of the bulk density of the developer in the stationary state, the flowing state, or in the state of being left intact, because of the individual difference in its shape. In addition, the fluctuation of the bulk density is greater due to the changes in the toner shape when the toner is in use for a long time.
The other one of them is the phenomenon related to the structure of the developing device. The developing sleeve in general use is arranged to rotate in the regular direction with respect to the photosensitive drum. Then, in order to prevent the uneven coating of the developer on the developing sleeve, the developer should be gathered in the vicinity of the regulating blade of the developing sleeve, and the structure should be arranged to compress the developer. Therefore, the longer the developing device is in use, the firmer the developer is compressed progressively.
In FIG. 5, the developing sleeve 25 rotates in the regular direction with respect to the photosensitive drum 3. As a result, it becomes necessary for the developing sleeve 25 to scoop up the developer from the developer container 18a by the function of the magnet 29. The magnet 29 is one having high magnetic force on two N magnetic poles and two S magnetic poles, respectively. Then, with the function thereof the developer 22 is scooped up. As a result, a magnetic binding force becomes stronger between the developing sleeve 25 and the regulating blade 28, thus compressing the developer 22 mechanically and magnetically. As a result, the bulk density of the developer is caused to change due to the change in the toner shape, or the bulk density of the developer is caused to change due to the external additives which are buried in it. Along with such changes, a change in magnetic permeability may take place within a predetermined volume of the developer after all.
Now, therefore, with the structure described above, the developer 22 is jammed into the collecting portion 16 of the developer in the vicinity of the regulating blade 28 for the developing sleeve 25, and the friction force between the developers themselves is increased by the rotation of the developing sleeve 25 if the developer is in a state where it is easily compressed. The more the developing sleeve 25 rotates, the more the amount of the electrostatic charge of the toner is increased. Thus, the change in the amount of the electrostatic charge of the toner becomes greater with respect to the initial amount of the electrostatic charge of the toner.
Therefore, it is necessary to change the control voltage that controls the intensity of the magnetic field generated by the toner concentration sensor in accordance with the environment, and also, to optimize the developer, as well as the structure of the developing device, for the further stabilization of the controlling method of the detected value of the toner concentration sensor.
It is an object of the invention to provide a developing device and an image forming apparatus, which are capable of controlling a toner concentration exactly.
It is another object of the invention to provide a developing device and an image forming apparatus, which are capable of suppressing a change in a bulk density and a change in an amount of the electrostatic charge of the developer to execute the exact toner concentration control.
It is still another object of the invention to provide a developing device which comprises:
(a) a developer bearing member for bearing and conveying a developer having a toner and carrier;
(b) a first magnetic pole arranged in the developer bearing member;
(c) a second magnetic pole arranged in the developer bearing member, wherein a polarity of the second magnetic pole is opposite to a polarity of the first magnetic pole, and the second magnetic pole is adjacent to the first magnetic pole, and arranged on a downstream side of the first magnetic pole in a moving direction of the developer bearing member;
(d) a regulating member for regulating a layer thickness of the developer borne by the developer bearing member, wherein, the regulating member is arranged in the vicinity of the second magnetic pole; and
(e) a concentration detector for detecting a concentration of the toner in the developer.
Also, it is a further object of the invention to provide an image forming apparatus which comprises:
(a) a developer bearing member for bearing and conveying a developer having a toner and carrier;
(b) a first magnetic pole arranged in the developer bearing member;
(c) a second magnetic pole arranged in the developer bearing member, wherein a polarity of the second magnetic pole is opposite to the polarity of the first magnetic pole, and the second magnetic pole being adjacent to the first magnetic pole, and arranged on a downstream side of the first magnetic pole in a moving direction of the developer bearing member;
(d) a regulating member for regulating a layer thickness of the developer borne by the developer bearing member, wherein the regulating member is arranged in the vicinity of the second magnetic pole; and
(e) a concentration detector for detecting a concentration of the toner in the developer.
Other objectives and advantages beside those discussed above will be apparent to those skilled in the art from the description of a preferred embodiment of the invention which follows. In the description, reference is made to accompanying drawings, which form a part hereof, and which illustrate an example of the invention. Such example, however, is not exhaustive of the various embodiment of the invention, and therefore reference is made to the claims which follow the description for determining the scope of the invention.